In present cone type and other speakers, undulations occur in a characteristic curve of sound pressure level versus frequency. The undulations occur due to the shape of the speaker even though the diaphragm vibrates reciprocally. For instance, in a speaker having a vertex angle of 120 degrees and diameter of 8 inches, there is a peak of approximately 5 dB around 1 KHz, and a dip of approximately 10 dB around 6 KHz. The peak and dip are reduced as the vertex angle becomes large, and therefore, it is necessary to make the sound radiating surface flat in order to remove the influence of the shape. However, the resonant frequency of a speaker diaphragm, which is flat and has a constant thickness, is low compared to that of a cone type speaker. For instance, there is a drawback such that the first order resonant frequency of a cone type speaker of vertex angle of 120 degrees, thickness of 0.5 millimeters, and diameter of 8 inches, resides between 1 and 2 KHz, while the first order resonant frequency of a flat speaker diaphragm of the same thickness resides between 100 and 200 Hz. Therefore, when a flat speaker diaphragm is used, it is required to make both the diaphragm thickness and the flexural rigidity great. In addition it is desirable to manufacture a speaker diaphragm with a low density material in order to make the weight of the speaker diaphragm light. For this reason a low density material foam resins, such as foam styrene the density of which is between approximately 0.015 and 0.1 g/cm.sup.3, have been used hitherto. However, when a speaker diaphragm is simply made of foam styrene or the like, it is necessary to use a high density material in order to obtain an adequate frequency range of the reciprocal movement thereof since the density of foam styrene has a proportional relationship with Young's modulus. Thereby the advantage of using a light foam resin for the speaker diaphragm may be lost and a necessary sound pressure level will not be obtained. To the contrary, if it is intended to obtain an adequate sound pressure level, it is necessary to use a foam resin of low density for the speaker diaphragm so that the frequency range of the reciprocal movement becomes narrow.
Although a speaker diaphragm assembly manufactured by adhering a sheet of paper to the sound radiating surface of a diaphragm was proposed in order to eliminate the above-mentioned drawbacks, an adequate adhesive force between the foam resin and the sheet of paper cannot be obtained with this speaker diaphragm assembly so that the sheet of paper easily comes off; an adequate stiffness cannot be obtained on the surface thereof; and the frequency range of the reciprocal movement cannot be satisfactorily widened.
Therefore, the present inventors tried to remove the above-mentioned drawbacks by changing the density of the foam resin as a function of location. The inventors made a speaker diaphragm with a foam resin, in such a manner that the density of the foam resin at the center portion of the diaphragm is greater than the density at the remaining portions, and the density decreases toward the periphery of the diaphragm. Although it was possible with such a speaker to expand the reproducible frequency range since a plurality of partial vibrations occurred in the middle and high frequency ranges, it was impossible to widen the frequency range of the reciprocal movement of the entire diaphragm. In other words, while it appeared that such a speaker having a low density foam resin at the sound radiating surface thereof, would have a widened reproducible frequency range, the actual flat range of the sound pressure level to frequency characteristic curve was not improved at all. Consequently, it can be said that such a speaker is not suitable for hi-fi sound reproduction.
The conventional diaphragm simply made of a foam resin and the resulting conventional speaker having such a diaphragm have other disadvantages as follows:
Namely, in a speaker using a diaphragm made of a relatively weak foam resin, such as a foam styrene resin, a foam acrylic resin, a foam urethane resin or the like, an aluminum foil is often secured to the center portion of the side surface of a speaker diaphragm or a light metallic plate is attached to the entire side surface of the speaker diaphragm in order to protect the diaphragm. However, in the speaker of the first-mentioned construction, the state of vibration transmission is not desirable since the vibrations of the voice coil are transmitted via the voice coil bobbin to only a joint portion between the voice coil bobbin and the diaphragm so that the sound quality is not particularly good. Furthermore, since the diaphragm, to which the vibrations are transmitted via the voice coil bobbin, is made of a foam resin, the diaphragm is relatively weak, whereby the efficiency of the vibration transmission from the voice coil bobbin is poor. In addition, another drawback is that it is troublesome to adhere the aluminum foil only to the center portion of the side surface of the diaphragm. On the other hand, in the latter-mentioned speaker, although the above-mentioned drawbacks are reduced, since the light metallic plate is attached to the entire side surface of the speaker diaphragm, the weight of the vibrating system increases to reduce the vibration efficiency of the speaker diaphragm. As a result, an adequate sound pressure level cannot be obtained and the speaker suffers from poor sound quality.
Another drawback inherent to such a conventional speaker is that foam resin speaker diaphragm, has a poor heat-resisting characteristic; the diaphragm melts because a joint portion between the voice coil bobbin and the speaker diaphragm gets sufficiently hot in response to heat generated in the voice coil that is transmitted via the voice coil bobbin to the speaker diaphragm. For this reason, it is impossible to drive such a speaker with high power. Furthermore, since tinsel codes for establishing electrical connections between speaker terminals and the voice coil are disposed close to the damper, the tinsel codes and the damper abut against each other due to the vibrations resulting in the occurrence of rustles so that the sound quality becomes poor.